Water-soluble binders are becoming of increasing interest because of the fact that when they are applied water, which ecologically is entirely acceptable, is predominantly emitted into the environment as the solvent. There has therefore been no lack of attempts to develop high quality binders with water as the solvent. Thus, for example, water-dilutable or water-soluble systems are becoming increasingly available in the field of painting and do-it-yourself lacquers, and here in particular in wood coatings. There is also a great interest in aqueous binders in the industrial field of use. The stricter legal regulations on emission of organic solvents have particularly contributed towards this.
The water-soluble binders are usually alkyd resins which contain lateral free carboxyl groups and, after neutralization of the carboxyl groups with amines, become water-soluble (c.f e.g. AT-PS 180 407, DE-OS 1 917 162, DE-OS 2 323 546 or DE-AS 2 239 094). Neutralization with amines leads to a weakly alkaline pH of the aqueous binders which, in conjunction with the ester groups in the polyester molecule, has the effect of a low stability towards hydrolysis. This is also manifested in the limited storage stability of the aqueous lacquer solutions, which precipitate after the pH gradually falls.
These binders moreover frequently contain considerable proportions of water-miscible organic co-solvents, which escape together with the other volatile constituents--water and amine--during drying. The ecological advantage which these binders have on the basis of their water-solubility is in this way reduced. A decrease in the co-solvent content as a rule impairs the dilution properties of the binders. Lacquers prepared from these have only low binder contents.
Aqueous binders based on polyacrylate are also already known (c.f. e.g. EP-A-0 218 906, DE-OS 2 347 999, DE-OS 2 357 152, DE-OS 2 507 842 and DE-OS 2 531 802). The coating agents of these previous publications are those based on anionically modified polymers, i.e. polymers containing carboxylate groups, which, in combination with a hardening agent, in particular, are thermosetting.
All such coating agents are largely unsuitable, in particular, for coating wood, since wood lacquers should as far as possible be hardenable without heat treatment, and in particular since, when used for coating wood, the amines which are contained in the lacquer and form counter-ions to the incorporated carboxylate groups are neutralized by the vegetable tannins which are contained naturally in wood and are described in Ullmanns Encyclopadie der technischen Chemie (Ullmanns Encyclopaedia of Industrial Chemistry), 3rd edition, 1960, volume 11, page 593 et seq. as compounds which are of complicated structure and contain numerous phenolic hydroxyl and carboxyl groups. Coagulation of the binder consequently rapidly occurs on penetration into wood.